A smaller cylindrical portion having a clamp groove formed on an outer peripheral surface thereof has a cylindrical surface portion which extends straight in an axial direction, an inclined surface portion which extends, while being inclined, radially outwardly from an end portion of the cylindrical surface portion and a vertical wall surface portion which extends straight radially outwardly from an end portion of the inclined surface portion, the cylindrical surface portion, the inclined surface portion and the vertical wall surface portion being formed continuously sequentially in that order from a side wall surface of the clamp groove. A stress concentration on a specific location on the smaller cylindrical portion can be suppressed by virtue of the stress alleviating action of these cylindrical surface portion, the inclined surface portion and the vertical wall surface portion.